No IANA assignment and no known standard service — a round number people pick for ad-hoc listeners.
49999 is in the Dynamic/Private range that RFC 6335 excludes from registration, and Nmap's service database has no name for it. Its only notable property is that it is memorable, which is why it shows up as a hand-chosen port for temporary tunnels, test servers, and internal tooling. There is no protocol you can assume from the number; fingerprint whatever answers.
Expose with care
Whatever is here was chosen by a human, often for something temporary and unauthenticated. Verify what it is before allowing it anywhere.
$ nmap -Pn -sV -p 49999 TARGETSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.